1. Field of the Invention
The present invention generally relates to a clamping mechanism for a semiconductor substrate and a method for transferring a semiconductor substrate using the same.
2. Description of the Related Art
If a film is formed on a conventional apparatus using the wafer pin transfer method or an apparatus without wafer clamping mechanism, excited gases contact the edges and even back side of the wafer, thereby causing a film to form in these areas. If a film is formed on the back side of the wafer or in other unwanted areas, however, the back side, etc., must be cleaned after a film has been formed in order to prevent contamination of the apparatus in the subsequent steps, and this cleaning becomes particularly important in processes when a metal film is formed. Because of this cleaning, the productivity drops inevitably.
FIGS. 7(a) to 7(c) show partially enlarged schematic diagrams of a conventional structure using the wafer pin transfer method. FIG. 7(c) shows a condition when a wafer is being transferred, where a wafer 15 is placed on a robot hand 20 and transferred to a specified position on a susceptor 34 at the transfer position. A wafer lift pin 35 (normally there are three wafer lift pins) is running through a pin hole (through hole) provided in the susceptor 34 and projecting out. FIG. 7(b) shows a condition when wafer transfer has ended, where the robot hand 20 comes down and places the wafer 15 on the upper tip of the lift pin 35, while the susceptor 34 rises and the wafer is placed onto the susceptor 34 with the upper tip of the lift pin 35 remaining down, thereby completing the transfer. FIG. 7(a) shows a condition during processing, where the wafer 15 rises to the processing position while being placed on the susceptor 34, and a clamp 33 on a guide ring 32 placed on the projected part of a chamber wall 31 is caused to contact the outer periphery of the wafer 15, and in this condition the clamp 33 is raised so that the outer periphery of the wafer 15 is clamped. When being separated from the guide ring 32, the clamp 33 is placed at the outer periphery of the susceptor and the outer periphery of the wafer, to clamp the wafer by its outer periphery in order to reduce the forming of film around the edges and on the back side of the wafer. From the end of processing to wafer transfer, the same steps are followed in the reverse order.
However, in the atomic layer deposition process where multiple reactant gases are changed sequentially to form a film, pressure variation is particularly large in each of the gas change and purge steps and thus the flows of gases from the transfer chamber to the reaction chamber through the through holes for wafer lift pins cannot be prevented. Because of this, problems such as increased gas change time and generation of particles also occur on ALD apparatuses where gases must be changed quickly. Furthermore, when the conventional clamp is used it is necessary to consider A (margin for the chamber wall and guide ring)+B (margin for the guide ring and clamp)+C (margin for the clamp and susceptor)+D (margin for the susceptor top and wafer)+E (transfer accuracy of the robot) to achieve proper positioning with the wafer on the susceptor. Since each part is made of a different material and subject to different temperatures, a margin of approx. 1 mm is needed when the expansion coefficients, machining accuracies and safety factors of respective parts are considered, and therefore a structure becomes necessary where the outer periphery of the wafer is clamped by 1.5 mm or more to completely prevent forming of film on the back side. In this case, the worst level of positional deviation can create an area with a dimension of anywhere from 2.5 mm at the maximum to 0.5 mm at the minimum around the outer periphery of the wafer where film is not formed, and this reduces the in-plane uniformity.